But while the deal would yield benefits in the short term, the longer term consequences of a Google tie-up could be disastrous for Yahoo, and Ballmer spends much of his letter arguing that would be the case.

An outsourcing deal would send a confusing message to Yahoo advertisers and prevent Yahoo from offering clients the benefits of a unified platform for both display and search advertising, Ballmer wrote.

He states convincingly that engineers working on Yahoo's ad systems would head for the door, and that regulatory and legal problems would rain down on Yahoo and any company that acquired it.

Even more horrifying for Ballmer, the deal would empower Google even more in search advertising, where it is already dominant.

"Accordingly, your apparent plan to pursue such an arrangement in the event of a proxy contest or exchange offer leads me to the firm decision not to pursue such a path," Ballmer wrote.

In other words, had it not been for the prospect of an outsourcing deal with Google, tonight's announcement would have likely been that Microsoft was getting ready to attempt a hostile takeover.

Thus, Google has helped accomplish what it wanted ever since Microsoft announced its intention to acquire Yahoo: to see the bid fizzle out. And, by extension, Google manages, again, to disrupt Microsoft's plans to give its Internet business a quick and major boost.

The question now is: How many chairs will Ballmer need to throw this weekend to vent his anger at this latest intrusion by Google?